The present invention generally relates to detection circuits and more particularly to a detection circuit for detecting amplitude modulated signals.
Detection circuits for detecting amplitude modulated signals are used in various electronic devices. In the simplest case, the detection circuit comprises a single diode. In the case of the detection of video signals and the like, a detection circuit that provides a particularly stable detection characteristic is required.
FIG. 1 shows a conventional detection circuit used in video signal processing apparatus for detecting video signals.
Referring to FIG. 1, a pair of npn transistors Q.sub.1 and Q.sub.2 are provided to form a differential amplifier. More specifically, the transistor Q.sub.1 has a collector connected to a voltage source V.sub.CC, an emitter connected to the ground via a constant current source I, and a base connected to an input terminal 10 25 via a coupling capacitor C1. The transistor Q.sub.2 has a collector connected commonly with the collector of the transistor Q.sub.1 to the voltage source V.sub.CC, an emitter connected, commonly with the emitter of the transistor Q.sub.1 to the ground via the constant current source I. Further, the base of the transistor Q.sub.1 is biased by a reference voltage source V.sub.REF via a resistor R.sub.1. Similarly, the transistor Q.sub.2 has a base biased by the same reference voltage source V.sub.REF via a resistor R.sub.2. The output of the detection circuit is obtained at an output terminal 11 which is connected to the emitter of the transistor Q.sub.1.
In operation, the transistors Q.sub.1 and Q.sub.2 are biased to produce a constant output at the output terminal 11 as long as the input signal at the input terminal 10 is below a predetermined bias voltage level. When there is an amplitude modulated radio frequency signal coming in at the input terminal 10, the voltage level at the base of the transistor Q.sub.1 is changed positively and negatively about the bias voltage level. Thereby, only the positive half cycle of the radio frequency signal is passed through the transistor Q.sub.1 from the base to the emitter after rectification at a p-n junction formed between the base and the emitter of the transistor Q.sub.1, and supplied to the output terminal 11. In the case that the radio frequency signal is in the negative half cycle, the output at the output terminal 11 remains stationary because of the rectifying action of the p-n junction.
FIG. 2 shows the operational characteristic of the detection circuit of FIG. 1 which shows the input versus output characteristic for the input signal V.sub.in in the vicinity of the bias voltage represented by V.sub.B.
Referring to FIG. 1, the output of the detection circuit increases generally linearly when the voltage level in the input signal exceeds the predetermined bias voltage to the transistor Q.sub.1 while it remains substantially stationary when the voltage level in the input signal is below the predetermined bias voltage. Thereby, the input signal at the input terminal 10 is rectified and the detection of the amplitude modulated signal is achieved.
In such a circuit, however, there is a problem in that the actual characteristic illustrated in FIG. 2 by a broken line is deviated from the ideal characteristic which is illustrated by a solid line. It should be noted that the ideal characteristic has a sharp deflection at the origin corresponding to the bias voltage V.sub.B and the output voltage at the output terminal 11 increases linearly with increasing voltage level at the input terminal 10. In the actual case, however, there is no such a sharp deflection in the characteristic but the characteristic is curved as illustrated schematically. This deviation is caused by the well known rectifying characteristic of the p-n junction formed between the base and the emitter of the transistor Q.sub.1.
When there is such a deviation, the output signal obtained at the output terminal 11 is inevitably distorted because of the non-linearity in the characteristic curve. Further, a small, negative output appears even when the input amplitude modulated signal is in the negative half cycle. It should be noted that such negative output should not appear in the rectified signal at the output terminal 11. Because the non-linearity in the rectifying characteristic is limited in the vicinity of the voltage V.sub.B at the base of the transistor Q.sub.1, it should be noted that such a distortion appears particularly conspicuous when the amplitude of the input signal V.sub.in at the input terminal 10 is small.
In order to avoid the problem of distortion of the output signal of the detection circuit, an amplifier circuit is usually provided before the detection circuit for amplifying the input radio frequency signal so that the effect of the distortion is minimized. However, such a measure is not desirable, as such an amplification at the earlier stages in the processing of signals may cause an unwanted saturation in the later stages. Thereby, an unwanted distortion is inevitably introduced.
It is much more desirable to detect the radio frequency signals prior to the amplification so that the problem of distortion caused by the amplification in the later stages is avoided. For this purpose, the distortion caused in the radio frequency signals of the minute amplitude due to the deviation in the detection characteristic as explained has to be eliminated.